Many utilities bury pipes and cables (“utility conveyances” or “conveyances”) underground for reasons of both safety and aesthetics. Underground burial often provides protection to utility conveyances against weather and other sources of potential damage. Utilities that undertake burial of their conveyances usually make extensive efforts to plot the location of each buried conveyance on a map to facilitate its location in case of repair or replacement. While a map will indicate the general location of a buried conveyance, more precise location information often becomes necessary, particularly in urban environments. For that reason, most utilities that bury their conveyances underground rely on electromagnetic signaling techniques to precisely locate such conveyances.
U.S. Pat. No. 5,644,237, issued Jul. 1, 1997 and assigned to the same assignee as the present application, describes a principle for electromagnetic signaling for locating a buried utility conveyance. To locate a buried conveyance, a locating tone (signal) is applied to a metallic component of the conveyance. In the case of a fiber optic cable, the metallic component is typically a metallic sheath or a copper tracer wire within the cable. Using a signal detector of a type well known in the art, a technician detects the locating tone radiated above ground to precisely locate the conveyance.
Utility conveyances often share rights of way. For example, fiber optic cables may run parallel to high voltage power lines, or may cross high voltage lines. Those fiber optic cables may contain metallic components running along their length, as noted, to assist tracing the cable underground. In such situations, special precautions must be taken to avoid 60 Hz current in the metallic component induced by the power lines. Furthermore, optic cables with metallic components must be protected from ground currents produced by lightning strikes and from other high energy power surges that may occur in the field.
One means for minimizing the current on the cable and preventing lightning damage to the buried cable is the installation of filter/arresters at various locations along the cable route. The filter/arresters are typically installed at cable splice locations. Those filters direct 60 Hz current to ground but let the frequencies used for cable locating pass. The filters also act to send to ground any voltage surge that may enter the cable. That surge may be a lightning strike, failed phase on the parallel high voltage line, etc. More than one filter may be required at one location due to the extreme voltage that may be induced.
The filters may fail after a period of time. A typical failure mode causes the filters to permit all frequencies to pass to ground, including the frequency of the locating tone. When that happens, the cable locating tone is prevented from passing the point of the failed filter. Cable location from that point is impossible.
When a technician determines that there is a potential problem with an arrester/filter, the technician must currently travel to each splice location and must measure the cable locating current approximately one hundred feet on each side of the splice. If the technician detects a significant current drop, the technician excavates the splice and replaces the filter. If multiple filters have been installed at a single splice, then the technician must identify the particular filter that failed. That is often not possible due to the functionality of the tone oscillator. In those cases, all filters at that location are replaced.
It is therefore desirable to provide a method and system for readily locating a failed filter/arrester in an underground cable system having many filter/arresters along its length. A method and system is furthermore needed for identifying a failed filter/arrester at a splice location where there are multiple filter/arresters installed. Such a system would minimize the fault-finding process and associated travel time.